Optical Fibers in commercial systems have been traditionally held by using a combination of pieces.
A connector assembly 100, such as shown in FIG. 1 as an exploded view is used to attach various fiber pieces (or fiber pieces and modules) together. A ferrule 102 is the part of the connector 100 into which the fibers 104 themselves are inserted before the ferrule 102 is inserted into the overall connector itself. The ferrule 102 is a ‘high-precision’ piece of the assembly 100. It holds the fiber(s) 104 in a precise position and ensures that when two connector pieces are attached, that the fibers in the two pieces are held in accurate alignment. The remainder of the connector 106 is ‘low precision’ relative to the ferrule 102.
In the multi-fiber connectors available today, most of the connections are for fiber arrays of 2 or more fibers, such as shown in U.S. Pat. No. 5,214,730, up to arrays of 1×12 (although some commercial 2×12 configurations have been tried). The connectors employed are referred to by various names depending upon who makes them. In 1×2 arrays, connectors are referred to as ST, LC, MT-RJ connectors while for 1×12 arrays the connectors are referred to as MTP®, MPO, and MPX connectors, among others. In the 1×12 or 2×12 area, all of the various connectors use a common type of ferrule commercially available from, among others, US Conec Ltd. and Alcoa Fujikura Ltd. In addition, commercial connectors for small arrays (less than 12) fibers have also been proposed, for example, in U.S. Pat. No. 5,743,785.
Fiber holding pieces, such as ferrules 102, can be made by molding plastic or epoxy pieces containing holes 108 into which optical fibers 104 can be inserted. Fibers must be able to be centered in each hole precisely and repeatably.
When an array of holes is made in a material for holding optical fibers, there are two aspects which need to be controlled. The spacing between holes (the “pitch” of the holes) and the diameter of each hole. Both have some margin of error due to the inherent inaccuracies of the fabrication techniques. If inaccuracies introduce errors in either (or both) pitch or size that are too large, then the fibers can be inserted at an angle or will not be positioned correctly in the ferrule. In either case, this negatively affects the ability to couple light efficiently, if at all, from one bundle to another or from an optical or opto-electronic component to a fiber bundle. If the hole pitch is inaccurate, then fibers from one bundle will not line up well with fibers of another bundle. However, even if the center-to-center pitch of the holes is very accurate, because the hole diameter is larger than the fiber (and each hole likely varies across an array) each fiber need not be in the exact same place in the hole as the other fibers in their holes, then that can cause misalignment, leading to inefficiencies or unacceptable losses. For example, if each of the holes in a ferrule piece was accurate to within 4 microns, then adjacent fibers could be off in pitch by up to 4 microns, since one fiber could be pushed to one side by 2 microns and the adjacent fiber could be pushed in the other direction by 2 microns. While this may be acceptable for multi-mode fibers, for single mode fibers this would be a huge offset that could make connections unacceptable or impossible.
In addition, fibers should generally not be placed in a hole at an angle or, if inserted at an angle, the particular angle should be specifically controlled.
FIG. 2 shows an example ferrule hole 200 and fiber 202. The inner circle, represents an actual fiber 202 while the outer circle, represents the hole 200 in the ferrule. As shown, the difference in sizes is not to scale but is exaggerated for purposes of illustration. Nevertheless, in actuality, the ferrule hole 200 must be larger than the fiber 202 by enough of a margin to allow for easy insertion—ultra-tight tolerances can not be effectively used. While the fiber 202 should ideally be centered with respect to the hole 200, as can be seen in FIG. 3, any individual fiber 202 could also be pushed in any hole 200 to somewhere else in the hole, for example, either the left or right edge (or any other edge) where it would not be centered within the hole 200. Thus, even if the ferrule has an accurate pitch “P” between hole centers 206, adjacent fibers 202 in an array may have an incorrect pitch “P+2ΔP” due to the offset P between the center 206 of each hole 200 and where the fiber 202 lies within the hole 200, in this case, causing an incorrect pitch of P plus 2 times the individual offset ΔP in each hole.
The 1×12 and 2×12 ferrule technology currently in commercial use is based upon a glass filled epoxy resin (a high-performance plastic) which is fabricated using a common plastic molding technique called transfer molding. Today, ferrules molded out of epoxies or plastics can be made to the necessary tolerances for multimode fibers, but special care must be taken during fabrication. Plastic molding technology is very process sensitive and molds having the requisite precision are extremely difficult to make. Even so, yields tend to be poor due to the inherent manufacturing process errors that occur in plastics molding. Since the tolerances on these pieces must be very accurate (on the order of about 1 to 2 micrometers), high yield manufacture is difficult. As a result, the cost of terminating fiber bundles into these connectors can be quite expensive, running hundreds of dollars per side. In addition, the process is not scalable to larger numbers of fibers (particularly 30 or more) because of inaccuracies and yield issues associated with molding technology and reliable production of ferrules for similar numbers of single mode fibers is even more difficult.
There has been an increasing need among users in the fiberoptic field for larger groups of fibers, so demand for connectors to handle these groups has been increasing as well. As a result, creation of connectors for larger arrays, such as 5×12, have been attempted. One manufacturer is known to have made a 5×12 connector array, but achieved such poor yields that they deemed an array of that size unmanufacturable. Moreover, the cost of producing the pieces resulted in their being sold for $500 each, due to poor yield, and the mold for producing the pieces was destroyed during the process.
The problem is that in plastic molding pieces for holding higher fiber counts in small spaces results in less structural integrity for the molded piece. As such, the prior art has been forced to do without commercial connectors for such large arrays, because 5×12 arrays can not be reliably created and commercial connectors for larger format arrays (e.g. even a 6×12) are considered prohibitively difficult to even attempt.
The ferrule area is very small, since ferrules for the above MTP, MPO, MPX connectors are about 0.07″ high, 0.3″ wide and 0.4″ deep, so molding or machining of features in the ferrules of the sizes required to hold multiple optical fibers (which typically have about a 125 micron diameter for a multimode fiber and a 9 micron diameter core for a single mode fiber) is very difficult. Since single mode fibers have an even smaller diameter than multimode fibers, molding or machining ferrules to accommodate large arrays of single mode fibers is currently, for all practical purposes, impossible—particularly on a cost effective commercially viable scale.
Additionally, making ferrules for arrays is made more difficult due to process variations during production because, as the holes approach the edge of the ferrule, the structural integrity of the walls decrease causing parts to have poor tolerance at the periphery, become overly fragile causing component collapse in some cases, or prohibiting removal of material from the inside of the piece that impedes or prevents fiber insertion.
Some have attempted to make two-dimensional fiber bundle arrays for by creating a dense packing of fibers together, for example, as described in U.S. Pat. No. 5,473,716, and K. Koyabu, F. Ohira, T. Yamamoto, “Fabrication of Two-Dimensional Fiber Arrays Using Microferrules” IEEE Transactions on Components, Packaging and Manufacturing Technology—Part C, Vol 21, No 1, January 1998. However, these attempts have not yielded a solution, particularly for the types of connectors mentioned above, because the inaccuracies of fiber production result in diameters of fibers which fluctuate within a 2 micron range (i.e. plus or minus 1 micron). Hence if 12 fibers are stacked in a row, there could be as much as 12 microns of inaccuracy in fiber alignment. Even with multi-mode fibers (the best of which use 50 micron cores), a misalignment of 12 microns will cause unacceptable light loss for most applications. For single mode fibers, which typically have 9 micron diameter cores, a 7 to 12 micron misalignment could mean that, irrespective of the alignment of the fiber at one end of the row, entire fibers at or near the other end of the row could receive no light whatsoever. For two-dimensional fiber arrays, the problem is even worse because the inaccuracy of the fiber is not limited to one direction. Thus, for example with a 16×16 array, a plus or minus 1 micron inaccuracy could result in fiber misalignments by up to 23 microns or more. Compounding the problem is the further fact that fiber inaccuracies stated as plus or minus 1 micron do not mean that fiber manufacturers guarantee that the fiber will be inaccurate by no more than 1 micron. Rather, the inaccuracy statement represents a standard deviation error range. This means that most of the fiber should only be that inaccurate. Individual fibers, or portions thereof, could have larger inaccuracies due to statistical variations.
As a result, the larger the number of fibers, the more likely a problem due to fiber inaccuracy will occur because, for example, using the 16×16 array above, the array would have 256 times the chance (because there are 16×16=256 fibers) of having at least one of these statistically anomalous fibers in the group.
Others have attempted to align two dimensional arrays of fibers (e.g. 4×4 arrays) in a research setting, but none have applied their techniques to conventional connector technologies. Moreover, the techniques are not suitable or readily adaptable for high yield, low cost, mass production as demanded by the industry. For example, some groups have examined the use of micromachined pieces made out of polyimide as described in J. Sasian, R. Novotny, M. Beckman, S. Walker, M. Wojcik, S. Hinterlong, “Fabrication of fiber bundle arrays for free-space photonic switching systems,” Optical Engineering, Vol 33, #9 pp. 2979–2985 September 1994.
Others have attempted to use silicon as a ferrule for precisely holding fiber bundle arrays since silicon can be manufactured with very high precision (better than 1 micron) and techniques for processing of silicon for high yield is, in general, well understood.
Early attempts at silicon machining for two-dimensional array fiber placement were performed with some limited success and one-dimensional fiber arrays, using fibers placed in V-Grooves etched into a piece of silicon, have been created, for example, as shown in FIG. 4A. The approach used the silicon pieces to hold the fibers but no attempt was made to integrate such an arrangement into a commercial connector.
Other groups took the V-Groove approach of FIG. 4A and performed an experiment where they stacked two of pieces together FIG. 4B for insertion into a connector. This resulted in a minimal array with two rows of fibers, as described in H. Kosaka, M. Kajita, M. Yamada, Y. Sugimoto, “Plastic-Based Receptacle-Type VCSEL-Array modules with One and Two Dimensions Fabricated using the self-Alignment Mounting Technique,” IEEE Electronic Components and Technology Conference, pp. 382–390 (1997), but the technique was not scalable to larger format two-dimensional arrays, such as shown in FIG. 4C.
Still other groups looked at holding larger format two-dimensional arrays using silicon pieces machined using wet-etching techniques, as described in G. Proudley, C. Stace, H. White, “Fabrication of two dimensional fiber optic arrays for an optical crossbar switch,” Optical Engineering, Vol 33, #2, pp. 627–635, February 1994.
While these silicon pieces were able to hold fibers, they were not designed to be, and could not readily be, used with existing ferrule or connector technology. Moreover, they could not be used for single mode fibers with any accuracy.
Thus, none of the above attempts have provided a viable solution to the problem of how to effectively create a large format fiber array which: allows for high precision holding of large arrays of fibers, especially single mode fibers, is compatible with current commercially used connectors that attach two fiber bundles to each other or one fiber bundle to a component containing an array of optical devices, such as lasers and/or detectors, and that allows for easy fiber termination in a rapid fashion at low production cost.
In addition, because of the above problems, there is presently no large format ferrule apparatus that can maintain fibers at a low angle, or at a precisely specified angle, for good optical coupling.
Collimating arrays are conceptually arrays of pipes for light. Mass production of collimating arrays for commercial applications has largely been dominated by the digital photographic camera and digital video camera world. These applications typically use a device called a “faceplate”, which is a multi-fiber assembly used to direct light onto optical detectors used for imaging. Since, for cameras, effective imaging requires the maximum amount of light reach the detectors, a faceplate will have several fibers per individual detector. In fact, in the most desirable faceplates, the number of optical fibers exceeds the number of optical detectors by many times. Thus, light being directed to a single detector in such a camera passes through multiple optical fibers arranged in parallel, and a camera has one detector per pixel. For imaging systems like cameras, this collimating technique is sufficient to accomplish its purpose. However, when dealing with optical communication systems, faceplates can not be used because the light loss resulting from such a collimating arrangement is significant. The faceplate technique (sometimes also referred to as oversampling) is also incompatible with the use of single mode fibers or lasers (which are highly desirable for use in high speed, long distance data transmission). Hence, the collimating technique of using a faceplate, such as made for use in cameras, is an unworkable approach for opto-electronic communication devices.
As noted above, for one-dimensional optical device arrays, attempts have been made to create collimators by using a piece of silicon wafer, into which V-Grooves are etched, and laying the fibers into the V-Grooves as shown in FIG. 4A. This is an operational approach for forming a one-dimensional array that is unsuitable for mass production.
Other groups have attempted to stack multiple V-Groove arrays on top of one another (FIGS. 4b, 4c) to create a larger collimating element. Unfortunately, the accuracy of stacking in the second dimension is limited by the accuracy of the thickness of the individual wafers, both on an absolute basis and on a relative basis, due to thickness variations over the area of the wafer. In addition, the stacked V-Groove technique requires such accuracy that individual stacks must be individually built up one at a time; a costly and inefficient process.
Presently, there are also no inexpensive two dimensional optical waveguide combiners available for commercial applications or that can be used with a fiber array. In some cases, optical fibers are twist fused to form a 2 to 1 “Y” branch, for example, for coupling a pumping laser to a single, signal carrying, fiber. For one-dimensional arrays of devices, Y branches have been created on the surface of a wafer by patterning, using lithographic techniques, to form waveguides. This technique provides robust control for a one-dimensional array, but cannot be extended into two dimensions since it is inherently a planar process.